1. Field of the Invention
This invention relates to an optical space communication apparatus for converting an optical signal modulated by a transmission signal into a beam of light and propagating the beam of light through the atmosphere to thereby effect communication between remote points.
2. Related Background Art
Generally, in communications utilizing optical signals, high-speed transmission of a large volume of information is possible and particularly, optical space communication using free space as a transmission path has an advantage that it abounds in portability as compared with wire communication as by optical fiber and its communication paths can be simply set up. In communication apparatuses according to the prior art, in order to improve the reliability of optical space communications, the correction, i.e., auto tracking, of the angle of a beam of light in the direction of emergence thereof is effected so that optical signals may not come off the apparatus.
FIG. 1 of the accompanying drawings shows the construction of an example of the prior art which is an optical space communication apparatus having the tracking function. On an optical path behind a lens 1 for transmission/reception, there are disposed a lens 2 and a movable mirror 3, and two actuators 4 and 5 for changing the angle of the movable mirror 3 are mounted on the movable mirror 3. These actuators 4 and 5 are adapted to be driven by the output of a tracking control circuit 6. On an optical path downwardly extending from the movable mirror 3, there are disposed a polarizing beam splitter 7 transmitting therethrough a polarized plane component parallel to the plane of the drawing sheet and reflecting a polarized plane component perpendicular to the plane of the drawing sheet by a cemented surface 7a, a lens 8, and a light emitting element 9 comprising a semiconductor laser or the like, and the outputs of an amplifier 10 and a transmission signal input end 11 are successively connected to the light emitting element 9.
On the other hand, on the optical path in the direction of reflection of the polarizing beam splitter 7, there are disposed a beam dividing mirror 12, a lens 13 and a light receiving element 14 having four light receiving elements 14a-14d as shown in FIG. 2 of the accompanying drawings. The output of the light receiving element 14 is connected to the tracking control circuit 6. On the optical path in the direction of reflection of the beam dividing mirror 12, there are disposed a lens 15 and a light receiving element 17 comprising an avalanche photodiode, a PIN photodiode or the like, and the outputs of an amplifier 18 and a reception signal output end 19 are successively connected to the light receiving element 17. There is also provided a collimation scope 20 made substantially parallel to the optical axis of the lens 1 and for an operator to visually confirm a partner apparatus.
During transmission, when a transmission signal is inputted from the transmission signal input end 11, it is amplified by the amplifier 10, and thereafter is outputted to the light emitting element 9. The light emitting element 9 intensity-modulates oscillated light in accordance with the input signal and converts it into an optical signal. The oscillated light from the light emitting element 9 passes through the lens 8 to the polarizing beam splitter 7, but this light is polarized in parallelism to the plane of the drawing sheet and therefore is intactly transmitted through the polarizing beam splitter 7, and is reflected leftwardly by the movable mirror 3, and is converted into a beam of light via the lenses 2 and 1, and emerges toward the partner apparatus.
During reception, the beam of light from the partner apparatus enters the lens 1 from left, is downwardly reflected by the movable mirror 3 and passes to the polarizing beam splitter 7. Since this beam of light is polarized in a direction perpendicular to the plane of the drawing sheet, it is rightwardly reflected by the cemented surface 7a of the polarizing beam splitter 7, and is divided into two directions by the beam dividing mirror 12. The beam of light reflected by the beam dividing mirror 12 is received by the light receiving element 17 and is converted into an electrical signal thereby, whereafter the electrical signal is amplified to a suitable level by the amplifier 18 and is outputted from the reception signal output end 19.
On the other hand, the beam of light transmitted through the beam dividing mirror 12 is condensed by the lens 13, and is received as a spot image S comprising a small circle as shown in FIG. 2 by the light receiving element 14. In the light receiving element 14, the outputs of the four light receiving elements 14a-14d are compared with one another to thereby find the position of the spot image S, which is outputted as a position signal to the tracking control circuit 6. On the basis of this position signal, the tracking control circuit 6 calculates the angle the beam of light from the partner apparatus forms with respect to the optical path of the host apparatus, and makes a driving signal for the actuators 4 and 5. The actuators 4 and 5 adjust the angle of the movable mirror 3 so that the spot image S may be received by the center of the light receiving element 14. Along therewith, the position of the light emitting element 9 is also adjusted and thus, the optical paths of the emergent beam of light and the incident beam of light coincide with each other, that is, the beam of light is accurately transmitted toward the partner apparatus. When during communication, the apparatus is inclined and the optical path of the received light deviates and the position of the spot image S on the light receiving element 14 deviates from the center, the movable mirror 3 is immediately moved and the optical path for the incidence of the beam of light is sequentially modified so that the spot image S may be received by the center of the light receiving element 14, thereby preventing the incident beam of light from deviating from the apparatus.
The above-described tracking function will not operate unless the beam of light from the partner apparatus arrives at a receivable level and the spot image S is received by a portion of the light receiving element 14. Accordingly, in the initial adjustment during the installation of the apparatus, the operator fixes the movable mirror 3 at an initial position near the midpoint, and manually effects the adjustment of the angle of a base 100 to thereby effect the adjustment of the direction of the entire apparatus while observing the partner apparatus by means of the collimation scope 20.
In the optical space communication apparatus, during communication, the beam of light is used in the state of a parallel beam of light. For example, when the beam diameter at the receiving point is set to the order of 2 m, the angle of expanse of the beam of light on the transmitting side is set to the order of 0.2.degree. for a transmission distance of 500 m, and is set to a small angle of expanse of the order of 0.06.degree. for a transmission distance of 2,000 m. Accordingly, when the adjustment of the direction of the entire apparatus is effected in the state of the same narrow angle of expanse of the beam of light as that during communication, a manual adjusting mechanism by the base 100 of high accuracy becomes necessary and the operation of adjusting the direction becomes cumbersome, and this leads to a high cost.
Also, whether the light receiving element 14 is receiving the spot image S can be confirmed by monitoring the light reception signal of the light receiving element 14, but whether the beam of light from the host apparatus is being received by the light receiving element of the partner apparatus cannot be confirmed from the host apparatus side. To ensure the beam of light to be received by the partner apparatus while observing the partner apparatus by means of the collimation scope 20 in this state, it is necessary that a value including the error of the angle of emergence of the beam of light and the visual confirmation error of the collimation scope 20 be suppressed to a small level. Therefore, not only requirements for the accuracy of the angle of the collimation scope 20 relative to the transmitting optical system during manufacture and the reproducibility of the midpoint position of the movable mirror 3 become severe, but also tolerance for the deviation of the collimation angle of the collimation scope 20 by a vibration, a shock, a variation with time or the like is quite limited even after manufacture, and this leads to difficulties in manufacture and high cost.